- ItemGeographic mosaics of fly pollinators with divergent color preferences drive landscape-scale structuring of flower color in Daisy communities(Frontiers Media, 2021-02-01) Ellis, Allan G.; Anderson, Bruce; Kemp, Jurene E.The striking variation in flower color across and within Angiosperm species is often attributed to divergent selection resulting from geographic mosaics of pollinators with different color preferences. Despite the importance of pollinator mosaics in driving floral divergence, the distributions of pollinators and their color preferences are seldom quantified. The extensive mass-flowering displays of annual daisy species in Namaqualand, South Africa, are characterized by striking color convergence within communities, but also color turnover within species and genera across large geographic scales. We aimed to determine whether shifts between orange and white-flowered daisy communities are driven by the innate color preferences of different pollinators or by soil color, which can potentially affect the detectability of different colored flowers. Different bee-fly pollinators dominated in both community types so that largely non-overlapping pollinator distributions were strongly associated with different flower colors. Visual modeling demonstrated that orange and white-flowered species are distinguishable in fly vision, and choice experiments demonstrated strongly divergent color preferences. We found that the dominant pollinator in orange communities has a strong spontaneous preference for orange flowers, which was not altered by conditioning. Similarly, the dominant pollinator in white communities exhibited an innate preference for white flowers. Although detectability of white flowers varied across soil types, background contrast did not alter color preferences. These findings demonstrate that landscape-level flower color turnover across Namaqua daisy communities is likely shaped by a strong qualitative geographic mosaic of bee-fly pollinators with divergent color preferences. This is an unexpected result given the classically generalist pollination phenotype of daisies. However, because of the dominance of single fly pollinator species within communities, and the virtual absence of bees as pollinators, we suggest that Namaqua daisies function as pollination specialists despite their generalist phenotypes, thus facilitating differentiation of flower color by pollinator shifts across the fly pollinator mosaic.
- ItemLarge uncertainties in future biome changes in Africa call for flexible climate adaptation strategies(John Wiley & Sons Ltd, 2020) Martens, Carola; Hickler, Thomas; Davis-Reddy, Claire; Engelbrecht, Francois; Higgins, Steven I.; Von Maltitz, Graham P.; Midgley, Guy F.; Pfeiffer, Mirjam; Scheiter, SimonAnthropogenic climate change is expected to impact ecosystem structure, biodiversity and ecosystem services in Africa profoundly. We used the adaptive Dynamic Global Vegetation Model (aDGVM), which was originally developed and tested for Africa, to quantify sources of uncertainties in simulated African potential natural vegetation towards the end of the 21st century. We forced the aDGVM with regionally downscaled high-resolution climate scenarios based on an ensemble of six general circulation models (GCMs) under two representative concentration pathways (RCPs 4.5 and 8.5). Our study assessed the direct effects of climate change and elevated CO2 on vegetation change and its plant-physiological drivers. Total increase in carbon in aboveground biomass in Africa until the end of the century was between 18% to 43% (RCP4.5) and 37% to 61% (RCP8.5) and was associated with woody encroachment into grasslands and increased woody cover in savannas. When direct effects of CO2 on plants were omitted, woody encroachment was muted and carbon in aboveground vegetation changed between –8 to 11% (RCP 4.5) and –22 to –6% (RCP8.5). Simulated biome changes lacked consistent large-scale geographical patterns of change across scenarios. In Ethiopia and the Sahara/Sahel transition zone, the biome changes forecast by the aDGVM were consistent across GCMs and RCPs. Direct effects from elevated CO2 were associated with substantial increases in water use efficiency, primarily driven by photosynthesis enhancement, which may relieve soil moisture limitations to plant productivity. At the ecosystem level, interactions between fire and woody plant demography further promoted woody encroachment. We conclude that substantial future biome changes due to climate and CO2 changes are likely across Africa. Because of the large uncertainties in future projections, adaptation strategies must be highly flexible. Focused research on CO2 effects, and improved model representations of these effects will be necessary to reduce these uncertainties.
- ItemAround the world in 500 years: Inter-regional spread of alien species over recent centuries(John Wiley & Sons Ltd., 2021) Seebens, Hanno; Blackburn, Tim M.; Hulme, Philip E.; Van Kleunen, Mark; Liebhold, Andrew M.; Orlova-Bienkowskaja, Marina; Pyšek, Petr; Schindler, Stefan; Essl, FranzAim: The number of alien species has been increasing for centuries world-wide, but temporal changes in the dynamics of their inter-regional spread remain unclear. Here, we analyse changes in the rate and extent of inter-regional spread of alien species over time and how these dynamics vary among major taxonomic groups. Location: Global. Time period: 1500–2010. Major taxa studied: Vascular plants, mammals, birds, fishes, arthropods and other invertebrates. Methods: Our analysis is based on the Alien Species First Record Database, which comprises >60,000 entries describing the year when an alien species was first recorded in a region (mostly countries and large islands) where it later established as an alien species. Based on the number and distribution of first records, we calculated metrics of spread between regions, which we termed “inter-regional spread”, and conducted statistical analyses to assess variations over time and across taxonomic groups. Results: Almost all (>90%) species introduced before 1700 are found in more than one region today. Inter-regional spread often took centuries and is ongoing for many species. The intensity of inter-regional spread increased over time, with particularly steep increases after 1800. Rates of spread peaked for plants in the late 19th century, for birds and invertebrates in the late 20th century, and remained largely constant for mammals and fishes. Inter-regional spread for individual species showed hump-shaped temporal patterns, with the highest rates of spread at intermediate alien range sizes. Approximately 50% of widespread species showed signs of declines in spread rates. Main conclusions: Our results show that, although rates of spread have declined for many widespread species, for entire taxonomic groups they have tended to increase continuously over time. The large numbers of alien species that are currently observed in only a single region are anticipated to be found in many other regions in the future.
- ItemChromosomal evolution in Raphicerus antelope suggests divergent X chromosomes may drive speciation through females, rather than males, contrary to Haldane's rule(Nature, 2021-02-04) Robinson, Terence J.; Cernohorska, Halina; Kubickova, Svatava; Vozdova, Miluse; Musilova, Petra; Ruiz-Herrera, AuroraChromosome structural change has long been considered important in the evolution of post-zygotic reproductive isolation. The premise that karyotypic variation can serve as a possible barrier to gene flow is founded on the expectation that heterozygotes for structurally distinct chromosomal forms would be partially sterile (negatively heterotic) or show reduced recombination. We report the outcome of a detailed comparative molecular cytogenetic study of three antelope species, genus Raphicerus, that have undergone a rapid radiation. The species are largely conserved with respect to their euchromatic regions but the X chromosomes, in marked contrast, show distinct patterns of heterochromatic amplification and localization of repeats that have occurred independently in each lineage. We argue a novel hypothesis that postulates that the expansion of heterochromatic blocks in the homogametic sex can, with certain conditions, contribute to post-zygotic isolation. i.e., female hybrid incompatibility, the converse of Haldane’s rule. This is based on the expectation that hybrids incur a selective disadvantage due to impaired meiosis resulting from the meiotic checkpoint network’s surveillance of the asymmetric expansions of heterochromatic blocks in the homogametic sex. Asynapsis of these heterochromatic regions would result in meiotic silencing of unsynapsed chromatin and, if this persists, germline apoptosis and female infertility.
- ItemThe invasive cactus Opuntia stricta creates fertility islands in African savannas and benefits from those created by native trees(2021-10-21) Novoa, Ana; Foxcroft, Llewellyn C.; Keet, Jan‑Hendrik; Pysek, Petr; Le Roux, Johannes J.The patchy distribution of trees typical of savannas often results in a discontinuous distribution of water, nutrient resources, and microbial communities in soil, commonly referred to as “islands of fertility”. We assessed how this phenomenon may affect the establishment and impact of invasive plants, using the invasion of Opuntia stricta in South Africa’s Kruger National Park as case study. We established uninvaded and O. stricta-invaded plots under the most common woody tree species in the study area (Vachellia nilotica subsp. kraussiana and Spirostachys africana) and in open patches with no tree cover. We then compared soil characteristics, diversity and composition of the soil bacterial communities, and germination performance of O. stricta and native trees between soils collected in each of the established plots. We found that the presence of native trees and invasive O. stricta increases soil water content and nutrients, and the abundance and diversity of bacterial communities, and alters soil bacterial composition. Moreover, the percentage and speed of germination of O. stricta were higher in soils conditioned by native trees compared to soils collected from open patches. Finally, while S. africana and V. nilotica trees appear to germinate equally well in invaded and uninvaded soils, O. stricta had lower and slower germination in invaded soils, suggesting the potential release of phytochemicals by O. stricta to avoid intraspecific competition. These results suggest that the presence of any tree or shrub in savanna ecosystems, regardless of origin (i.e. native or alien), can create favourable conditions for the establishment and growth of other plants.